1. Field of the Invention
The present invention relates to a display device and more particularly to an in-plane switching (IPS) type liquid crystal display device improved in reliability of a seal portion.
2. Description of the Related Art
A liquid crystal display device includes a TFT substrate on which pixel electrodes, thin film transistors (TFTs), and the like are formed in a matrix, a counter substrate which is disposed to face the TFT substrate and on which color filters and the like are formed at positions corresponding to the pixel electrodes of the TFT substrate, and liquid crystal interposed between the TFT substrate and the counter substrate. An image is formed by controlling the transmittance ratio of light through liquid crystal molecules for each pixel.
The liquid crystal display device is flat and light in weight, and therefore the application of liquid crystal display device has expanded in various fields from large display devices such as TVs to small display devices such as mobile phones or digital still cameras (DSCs). On the other hand, the liquid crystal display device has a problem of viewing angle characteristics. Viewing angle characteristics refer to a phenomenon where brightness varies or chromaticity varies between when a screen is seen from the front and when the screen is seen from a diagonal direction. The IPS type, in which the liquid crystal molecules are moved by a horizontal direction electric field, has excellent viewing angle characteristics.
In the IPS type, it is not necessary to form a pretilt angle for liquid crystal molecules in the vicinity of an alignment film. Therefore, an alignment axis for the alignment film can be formed not by a rubbing method but by a photo-alignment method. The photo-alignment has such an advantage that it does not cause static electricity compared to the rubbing method, for example.
The photo-alignment imparts anisotropy to an alignment film with the irradiation of polarized ultraviolet radiation so that liquid crystal molecules are aligned in a specific direction with respect to the alignment film. JP-A-2005-351924 describes a technique relating to the photo-alignment described above.
The photo-alignment is performed by irradiating an alignment film made of a polymer with ultraviolet radiation polarized in a specific direction. For example, when the polymer formed in a network is irradiated with polarized ultraviolet radiation, the polymer in a specific direction with respect to the polarized direction of ultraviolet radiation is damaged. This can form anisotropy for the alignment film for aligning liquid crystal molecules. There is no problem when only the alignment film is irradiated with polarized ultraviolet radiation for photo-alignment. However, when a portion other than the alignment film is irradiated, the irradiated portion is degraded by the ultraviolet radiation, causing problems.
IPS type liquid crystal display devices have been used for small liquid crystal display devices. Manufacturing small liquid crystal display devices one by one is inefficient. Therefore, a number of liquid crystal display devices are formed on a mother substrate to simultaneously manufacture a number of liquid crystal display devices.
FIG. 13 shows an example where 35 small liquid crystal display cells 1 are prepared on a mother substrate. A mother TFT substrate 1000 on which a number of TFT substrates 100 each having TFTs and pixel electrodes are formed, and a mother counter substrate 2000 on which a number of counter substrates 200 each having color filters and the like formed thereon are formed are aligned to each other. The mother TFT substrate 1000 and the mother counter substrate 2000 are bonded together with sealing materials 15 and a mother substrate sealing material 151. In FIG. 13, each of hatched rectangles surrounded by the sealing material 15 indicates a range where an alignment film 113 is formed.
Small liquid crystal display devices are required to be thin. For example, the TFT substrate and the counter substrate each has a thickness of about 0.2 mm. However, such thin glass does not exist as a standard product. Moreover, such a thin glass substrate cannot undergo the process at present. Accordingly, in a state of the mother counter substrate 2000 or the mother TFT substrate 1000, glass having a thickness of about 0.5 mm is used, and after the mother counter substrate 2000 and the mother TFT substrate 1000 are aligned to each other to form a mother substrate, the outer surface of the mother counter substrate 2000 or the mother TFT substrate 1000 is polished.
Polishing is often carried out with a combination of mechanical polishing and chemical polishing. In both mechanical polishing and chemical polishing, when abrasive enters the inside of the mother substrate, the liquid crystal cells 1 inside of the mother substrate become defective. Therefore, the inside of the mother substrate is protected by the mother substrate sealing material 151. The mother substrate sealing material 151 formed at the periphery of the mother substrate is sealed with a mother-substrate end-sealing material 161. The mother substrate shown in FIG. 13 is separated into individual liquid crystal cells after polishing. FIG. 14 shows the mother counter substrate 2000 constituting the mother substrate shown in FIG. 13, in which 35 counter substrates 200 are formed corresponding to the liquid crystal cells 1 in FIG. 13. FIG. 14 shows the mother counter substrate 2000 at a stage before forming the sealing materials 15 or the mother substrate sealing material 151 thereon. In FIG. 14, the alignment film 113 is formed on each of the counter substrates 200. Since the presence of the alignment film in a seal portion reduces the adhesive force of the sealing material 15, the alignment film 113 is formed so as to avoid the seal portion and cover a display region.
In FIG. 14, the alignment film 113 is formed by flexographic printing. After forming the alignment film 113, photo-alignment is performed on the alignment film 113 using polarized ultraviolet radiation. At this time, the entire surface of the mother counter substrate 2000 is irradiated with polarized ultraviolet radiation. This is because irradiation of the alignment films with polarized ultraviolet radiation one by one increases the manufacturing cost. Accordingly, also a portion where the alignment film is not formed is irradiated with polarized ultraviolet radiation.
FIG. 15 is a cross-sectional structure of one counter substrate 200 at an edge portion, showing a state of irradiation of polarized ultraviolet radiation for the photo-alignment. At the edge portion of the counter substrate 200, a light shielding film 202, a color filter 201, an overcoat film 203, and the like are formed as will be described later. The light shielding film 202 has a function to improve the contrast of a screen or to enhance the appearance of the screen periphery and is also referred to as black matrix. In the specification, however, the term “light shielding film” is used. As shown in FIG. 15, at a portion where the alignment film 113 is not present, a hatched portion 2031 of the overcoat film 203 is directly irradiated with ultraviolet radiation. Therefore, this portion 2031 of the overcoat film is degraded, whereby the overcoat film 203 allows moisture to easily penetrate.
FIG. 16 is a cross-sectional view of an edge portion of the counter substrate 200, showing a state where after performing the photo-alignment using polarized ultraviolet radiation, the sealing material 15 is formed. Since the hatched portion 2031 of the overcoat film 203 is degraded by ultraviolet radiation, moisture penetrates through this portion 2031 of the overcoat film to the surface of the light shielding film 202.
FIG. 17 is a cross-sectional view of an edge portion of a liquid crystal display panel in a state where the TFT substrate 100 and the counter substrate 200 are bonded together, and liquid crystal layer 300 is sealed therebetween. In FIG. 17, an inorganic passivation film 106, an organic passivation film 107, and the alignment film 113 are formed on the TFT substrate 100. The light shielding film 202, the color filter 201, the overcoat film 203, and the alignment film 113 are formed on the counter substrate 200. In FIG. 17, since the hatched portion 2031 of the overcoat film 203 of the counter substrate 200 is degraded by ultraviolet radiation in the photo-alignment, moisture easily enters this portion from the outside.
When moisture enters the degraded overcoat film 2031, the moisture reaches the light shielding film 202 and alters the light shielding film 202. Especially when moisture acts on the light shielding film 202, the adhesive force between the light shielding film 202 and the substrates 200 is reduced, which reduces the reliability in the seal portion. Moreover, when moisture acts on the light shielding film 202, the electrical resistance of the light shielding film 202 is reduced, an electric field in the liquid crystal layer 300 is disturbed by the influence of the light shielding film 202, and the contrast is reduced by light leakage.